Some of Iapetus’ dark surface interrupts the moon’s lighter terrain in this Cassini spacecraft view. Lapetus is the third-largest moon of Saturn, and eleventh in the solar system, discovered by Giovanni Domenico Cassini in 1671. Lapetus is best known for its dramatic ‘two-tone’ coloration, but recent discoveries by the Cassini mission have revealed several other unusual physical characteristics, such as an equatorial ridge that runs about halfway around the moon.

FIREBALL ON JUPITER: An amateur astronomer in Japan has video-recorded a fireball on Jupiter. This marks the third time in only 13 months that amateur astronomy buffs have detected signs of something hitting the giant planet. Will the latest impact leave behind a visible cloud of debris? .

SPACE STATION and SPACE PLANE FLYBYS: This is a good week for satellite watchers in the USA and Canada. Both the International Space Station and the US Air Force X-37B space plane are making a series of favorable passes over North American towns and cities. The spacecraft are easy to find using our Simple Satellite Tracker–an app for iPhones and Android. Visit http://simpleflybys.com for downloads and more information.

Sunlight passing through the Cassini Division between Saturn’s A and B rings sweeps across and illuminates the surface of the moon Janus in this movie captured shortly after Saturn’s August 2009 equinox.

This image released by NASA in Washington on September 9, 2009 shows Gravitational Lensing in Galaxy Cluster Abell 370 through The Hubble Space Telescope’s newly repaired Advanced Camera for Surveys (ACS). Abell 370 is one of the very first galaxy clusters where astronomers observed the phenomenon of gravitational lensing, where the warping of space by the clusterÕs gravitational field distorts the light from galaxies lying far behind it. This is manifested as arcs and streaks in the picture, which are the stretched images of background galaxies.

A gravitational lens is formed when the light from a very distant, bright source (such as a quasar) is “bent” around a massive object (such as a cluster of galaxies) between the source object and the observer. The process is known as gravitational lensing, and is one of the predictions of Albert Einstein’s general theory of relativity.

Although Orest Chwolson is credited as being the first to discuss the effect in print (in 1924), the effect is more usually associated with Einstein, who published a more famous article on the subject in 1936.

Fritz Zwicky posited in 1937 that the effect could allow galaxy clusters to act as gravitational lenses. It was not until 1979 that this effect was confirmed by observation of the so-called “Twin QSO” SBS 0957+561.